Bruce Wayne, aka Batman, has the most pungent line in “Justice League,” a wilted superhero salad from the DC Cinematic Universe. “What are your superpowers again?” the Flash asks him. “I’m rich,” Bruce replies. Vast riches have been heaped on this production, to benumbing effect. The battles to save the world are generic/titanic; the villain is a bloodless bore with a boom-box roar; and the screen, like the ragged story, is chockablock with such underdeveloped overachievers as Batman, Wonder Woman, Aquaman, Cyborg and the Flash. Of all the members of the newly constituted League, only the Flash, played by Ezra Miller...